1. Field of the Invention
The present invention relates generally to drilling a subterranean borehole and, more specifically, to drill bits for drilling subterranean formations and having a capability for drilling out structures and materials which may be located at or proximate the end of a casing or liner string, such as a casing bit or shoe, cementing equipment components and cement.
2. State of the Art
The drilling of wells for oil and gas production conventionally employs longitudinally extending sections or so-called “strings” of drill pipe to which, at one end, is secured a drill bit of a larger diameter. After a selected portion of the borehole has been drilled, the borehole is usually lined or cased with a string or section of casing. Such a casing or liner usually exhibits a larger diameter than the drill pipe and a smaller diameter than the drill bit. Therefore, drilling and casing according to the conventional process typically requires sequentially drilling the borehole using drill string with a drill bit attached thereto, removing the drill string and drill bit from the borehole, and disposing casing into the borehole. Further, often after a section of the borehole is lined with casing, which is usually cemented into place, additional drilling beyond the end of the casing may be desired.
Unfortunately, sequential drilling and casing may be time consuming because, as may be appreciated, at the considerable depths reached during oil and gas production, the time required to implement complex retrieval procedures to recover the drill string may be considerable. Thus, such operations may be costly as well, since, for example, the beginning of profitable production can be greatly delayed. Moreover, control of the well may be difficult during the period of time that the drill pipe is being removed and the casing is being disposed into the borehole.
Some approaches have been developed to address the difficulties associated with conventional drilling and casing operations. Of initial interest is an apparatus, which is known as a reamer shoe that has been used in conventional drilling operations. Reamer shoes have become available relatively recently and are devices that are able to drill through modest obstructions within a borehole that has been previously drilled. In addition, the reamer shoe may include an inner section manufactured from a material that is drillable by drill bits. Accordingly, when cemented into place, reamer shoes usually pose no difficulty to a subsequent drill bit. For instance, U.S. Pat. No. 6,062,326 to Strong et al. discloses a casing shoe or reamer shoe in which the central portion thereof may be configured to be drilled through. In addition, U.S. Pat. No. 6,062,326 to Strong et al. discloses a casing shoe that may include diamond cutters over the entire face thereof, if it is not desired to drill therethrough.
As a further extension of the reamer shoe concept, in order to address the problems with sequential drilling and casing, drilling with casing is gaining popularity as a method for initially drilling a borehole, wherein the casing is used as the drilling conduit and, after drilling, the casing remains downhole to act as the borehole casing. Drilling with casing employs a conventional drill bit attached to the casing string, so that the drill bit functions not only to drill the earth formation, but also to guide the casing into the wellbore. This may be advantageous as the casing is disposed into the borehole as it is formed by the drill bit, and therefore eliminates the necessity of retrieving the drill string and drill bit after reaching a target depth where cementing is desired.
While this procedure greatly increases the efficiency of the drilling procedure, a further problem is encountered when the casing is cemented upon reaching the desired depth. While one advantage of drilling with casing is that the drill bit does not have to be retrieved from the wellbore, further drilling may be required. For instance, cementing may be done for isolating certain subterranean strata from one another along a particular extent of the wellbore, but not at the desired depth. Thus, further drilling must pass through or around the drill bit attached to the end of the casing.
In the case of a casing shoe that is drillable, further drilling may be accomplished with a smaller diameter drill bit and casing section attached thereto that passes through the interior of the first casing to drill the further section of hole beyond the previously attained depth. Of course, cementing and further drilling may be repeated as necessary, with correspondingly smaller and smaller components, until the desired depth of the wellbore is achieved.
However, drilling through the previous drill bit in order to advance may be difficult, as drill bits are required to remove rock from formations and, accordingly, often include very drilling resistant, robust structures typically manufactured from materials such as tungsten carbide, polycrystalline diamond, or steel. Attempting to drill through a drill bit affixed to the end of a casing may result in damage to the subsequent drill bit and bottom-hole assembly deployed or possibly the casing itself. It may be possible to drill through a drill bit or a casing with special tools known as mills, but these tools are unable to penetrate rock formations effectively and the mill would have to be retrieved or “tripped” from the hole and replaced with a drill bit. In this case, the time and expense saved by drilling with casing would have been lost. One apparatus for avoiding tripping of a window mill used to drill through a whipstock set in casing is disclosed in U.S. Pat. No. 7,178,609, referenced above, from which priority is claimed and the disclosure of which is incorporated herein by reference. However, other approaches have been developed for use in other situations to allow for intermittent cementing in combination with further drilling.
In one approach, a drilling assembly, including a drill bit and one or more hole enlargement tool such as, for example, an underreamer, is used which drills a borehole of sufficient diameter to accommodate the casing. The drilling assembly is disposed on the advancing end of the casing. The drill bit can be retractable, removable, or both, from the casing. For example, U.S. Pat. No. 5,271,472 to Letumo discloses a drill bit assembly comprising a retrievable central bit insertable in an outer reamer bit and engageable therewith by releasable lock means, which may be pressure fluid operated by the drilling fluid. Upon completion of drilling operations, the motor and central retrievable bit portion may be removed from the wellbore so that further wellbore operations, such as cementing of the drillstring or casing in place, may be carried out or further wellbore extending or drilling operations may be conducted. Since the central portion of the drill bit is removable, it may include relatively robust materials that are designed to withstand the rigors of a downhole environment, such as, for example, tungsten carbide, diamond, or both. However, such a configuration may not be desirable since, prior to performing the cementing operation, the drill bit has to be removed from the wellbore and thus the time and expense to remove the drill bit is not eliminated.
Another approach for drilling with casing involves a casing drilling shoe or bit adapted for attachment to a casing string, wherein the drill bit comprises an outer drilling section constructed of a relatively hard material and an inner section constructed of a drillable material. For instance, U.S. Pat. No. 6,443,247 to Wardley discloses a casing drilling shoe comprising an outer drilling section constructed of relatively hard material and an inner section constructed of a drillable material such as aluminum. In addition, the outer drilling section may be displaceable, so as to allow the casing shoe to be drilled through using a standard drill bit.
Also, U.S. Patent Application 2002/0189863 to Wardley discloses a drill bit for drilling casing into a borehole, wherein the proportions of materials are selected such that the drill bit provides suitable cutting and boring of the wellbore while being able to be drilled through by a subsequent drill bit. Also disclosed is a hard-wearing material coating applied to the casing shoe as well as methods for applying the same.
However, a casing drilling shoe or bit as described in the above patent and application to Wardley may be unduly complex, require careful selection of combinations of materials including easily drillable materials and, thus, may be undesirably expensive to manufacture.
Casing bits as disclosed and claimed in U.S. Pat. No. 7,395,882, referenced above, from which priority is claimed and which is incorporated by reference herein, have addressed many of the deficiencies associated with the Wardley structures.
However, to enable the manufacture of a casing bit (or casing shoe) from a robust, inexpensive and easily worked material such as, for example, steel or other materials which are generally non-drillable by superabrasive cutting elements, it would be desirable to have a drill bit offering the capability of drilling through such a casing bit and, if employed, other components disposed in a casing or liner string thereabove as well as cement, yet offering the formation drilling capabilities of a conventional drill bit employing superabrasive cutting elements.